The present invention relates to a novel orthopaedic device, of the type formed by a support plate provided with at least one orifice for passage of a screw for fixing it to the receiving bone material, adapted in particular for being implanted in the cervical spine of a patient.
In case of intervertebral fusion (arthrodesis), a plate-shaped implant is generally used to consolidate the cervical spine and to ensure the position keeping of the bone graft placed by the practitioner between two vertebrae, in replacement of the intervertebral disk that has been extracted.
The corresponding plate is provided several orifices each intended to receive a fixing screw capable of anchoring in the receiving bone material. The shape and dimensions of this plate, and of the associated screws, are adapted to the treated site and to the implantation constraints.Very generally, the support plates are proposed to the practitioner with a batch of screws of different sizes, only some of which are used depending on the implantation site.
For optimum efficiency and security, at the time of implantation, the practitioner applies a particular tightening to each screw, so that the associated plate is suitably positioned and maintained against the vertebrae.
Most of the present orthopaedic devices comprise means for locking or clamping the screws, so as to prevent or at least limit these screws from moving back with respect to the associated plate, after the assembly is reloaded (the corresponding loosening of the screws being liable to create discomfort to the patient or to imply high risks of lesion).
Such orthopaedic devices are described, for example, in U.S. Pat. No. 7,001,389 and US-2004/0260306. In these documents, the fixing screws each comprise a head provided with an annular channel within which a radially elastic locking ring is pre-mounted. As for the plate, it is provided with orifices whose internal surface is provided with a groove delimited by an upper annual material strip having a beveled top face, and a lower face forming an annular flat surface.
In practice, each screw is implanted through one of the orifices of the plate and screwed until its head automatically locks within the associated orifice. To this end, when the screw head reaches the upper edge of the groove, its spring ring is radially compressed by the above-mentioned beveled top surface, then it automatically expanses within this groove to form the required stop for preventing back-moving or unscrewing, with the above-mentioned annular flat surface.However, with such devices, it is necessary to equip each screw with a locking ring, namely both the screws that are implanted and those that are not used, which entails a not insignificant cost.Moreover, once the screws implanted, it is not possible to remove them without damaging the material.
Another type of material of this sort is described in FR-2 778 088. Here, the screw backstop means consist of a slide slidingly mounted on the support plate, capable to partially cover the screw heads once the latter are suitably tightened.
However, this structure is rather complex. Moreover, this type of implant necessitates additional operating time to insert or remove the screws, as well as additional equipment.
Further, the backstop means are inevitably efficient only on a well-defined area of the screw periphery.
U.S. Pat. No. 6,602,255 presents still other implants of this sort. In a possible embodiment, the corresponding implant is formed by a support plate provided with insertion orifices for the fixing screws, within which orifices is provided a receiving groove for an open locking ring, the latter two having general shapes that are complementary with each other. Under the groove equipped with its locking ring, each orifice is provided with a portion having a female spherical peripheral surface.
The fixing screws comprise a head extended by a threaded body. These screw heads are equipped with an annular peripheral protuberance or excrescence, having a male spherical surface that is complementary with the female spherical surface of the associated orifice of the plate, said protuberance connecting with the upper part of said screw head by a circular flat surface perpendicular to the screw axis.During the implant setting, the fixing screws are tightened until the spherical surface of their annular protuberance is in contact with the complementary spherical surface of the support plate. Before this contact, the annular protuberance is forced through the slotted ring, making the latter radially expands within its receiving groove. After the upper circular flat surface of this protuberance has passed the slotted ring, the latter automatically recovers its rest position to form the required locking member (through plane contact between the annular flat surface of the screw and the lower face of the slotted ring).The spherical contact surfaces between the plate orifices and the screw heads offer the practitioner a possibility of angularly adjusting the screw.
However, given the structure of this implant, when the axis of the screw is not merged with that of its receiving orifice, the contact between the annular flat surface of the screw heads and the locking ring is not optimum, which minimizes the rest surface area that serves to prevent the screw from moving back.
Further, the removal of the screws after implantation is not very easy to make and necessitates particular additional equipment. This implies an additional cost as for the material, and also an additional operating time.